Polymer nanofibers can be produced from solution-based electrospinning or electroblowing, however, they have very high processing cost, limited throughputs and low productivity. Melt blowing nanofiber processes that randomly lay down fibers do not provide adequate uniformity at sufficiently high throughputs for most end use applications. The resulting nanofibers are often laid on substrate layer of coarse fiber nonwoven or microfiber nonwoven to construct multiple layers. A problem with melt-blown nanofibers or small microfibers, exposed on the top of the web, they are very fragile and easily crushed by normal handling or contact with some object. Also, the multilayer nature of such webs increases their thickness and weight, and also introduces some complexity in manufacture. Centrifugal spun nanofiber process has demonstrated lower manufacturing cost in massive nanoweb production.
U.S. Pat. No. 8,277,711 B2 to DuPont discloses a nozzle-less centrifugal melt spin process through rotational thin film fibrillation. The nanofibers with number average diameter less than about 500 nm have been disclosed and shown in examples spun from polypropylene and polyethylene resins. In practice, the operation window is very narrow for making the uniform nanofibers due to the requirement of uniform and smooth thin film flow on the inner surface of the spinning disk, which requires the right rheological properties of polymer and the right combination of the temperature, the rotating speed and melt feeding rate. Otherwise, there would not be uniform and smooth thin film flow on the inner surface of the spinning disk. Instability of the thin film flow and variation of the thickness in the thin film will cause the formation of larger fibers mixed with the nanofibers. If the disk temperature is too high, the molten state threads might lose elasticity due to the potential thermal degradation and the break down into droplets, resulting in nanofibers that might be mixed with the micro-particles or powders. If the disk temperature is too low, the shock-wave instability in the melt filming flow on the inner surface of spinning disk might cause the moving fronts of the filming flow broken off and throw off from the spinning disk, resulting in the nanofibers might be mixed with the large size defects, such as, the “tadpoles” and the “spatters”.
The nanofibers made from the process of U.S. Pat. No. 8,277,711 B2 can be laid on a belt collector to form uniform web media using the process of WO 2013/096672, in which the complicate air flow management needs to be implemented. Otherwise, the uniform web cannot be laid because of the swirling and the twisting of fiber stream due to the “tornado”-like effect under the high speed rotating disk.
U.S. Pat. No. 8,231,378 B2 to University of Texas (later the FibeRio Technology Corporation) discloses a centrifugal nanofiber spinning from rotating spinnerets with nozzles, such as, syringes, micro-mesh pores or non-syringe gaps with a typical openings of diameter sizes of 0.01-0.80 mm. The microfibers with the number average diameter of one micron or larger and the nanofibers have been shown. The nanofiber with number average diameter less than about 300 nm has been disclosed. In general, the centrifugal spinning through nozzles has much less throughput due to the capillary flow through the nozzle orifices and the melt die swell at the nozzle exit. For the current state of the art, the only very low basis weight of the thin layer nanofibers can be deposited on scrim when the polypropylene nanofiber spun from melt. The PP web has very low strength and difficult to handle without scrim.
What is needed is the improvement of centrifugal melt spun nanofiber process of U.S. Pat. No. 8,277,711 B2 to make the nanofibrous web in a much broad operation window, as well as, to address the potential thermal degradation in centrifugal melt spinning, to address the issues mentioned above and the elimination of the defects.